User blog comment:AssassinRome/Assassins in Japan?/@comment-5174994-20120806211639/@comment-4043223-20120806223927

Lower class peasantry wore black as they were mainly labourers (and as such clothing of a darker nature was the norm) and practicioners of ninjitsu wore similar attire to blend in when the majority (which at the time was the lower class). In order to signify status the gentry wore flashy colourful clothing and that would have been unsuitable. Also, so called ninjas mainly carried out their tasks at night so the dark color offered them better camouflage as well.